1. Field of the Invention
This invention relates to a structure of a synchronous motor with movable part having permanent magnets, and more particular to a motor structure which is preferable to a linear motor.
2. Description of the Related Art
FIG. 11 shows a cross section of a conventional vernier-type linear motor incorporating permanent magnets.
Numeral 3 is a slider which forms a movable part and 7 is one of the slots on the slider 3 indicated by S1 to S12. A bipolar three-phase alternating current winding, commonly used for such a rotation-type induction motor, is wound to be aligned in a line in each slot. FIG. 3 illustrates an example of their wire-wound diagram. R, S, and T are terminals of three-phase alternating current windings, and N is a neutral point. Numeral 15 is a tooth of the slider 3. The width of the front end of respective teeth 15 is almost the same as that of respective inlets of slots 7 so that their magnetic reluctance change between high and low alternatively according to a movement of the slider 3 in the direction shown by the arrow i.e. a side-to-side direction from a stator 1.
Numeral 1 is a stator and alternating poles N and S of permanent magnets are attached at regular intervals on its surface.
From Fleming's rule, the force F generated by a single turn of winding for a motor is generally derived as follows: EQU F=B.multidot.I.multidot.L
where B is magnetic flux density, I is electric current, and L is effective length of an electric wire. And the motor power P is represented as follows: EQU P=F.multidot.dX/dt
where X is the distance in the movement direction of the slider, dX/dt is the velocity of the slider.
Taking voltage here to be V, the power P is represented as follows: EQU P=V.multidot.I=d.phi./dt.multidot.I
where .phi. is flux linkage of a single turn of winding. By the neglect of variations in magnetic energy occurred within a linear motor, the both formulas above lead with, EQU P=F.multidot.dX/dt=d.phi./dt.multidot.I
As a consequence, the thrust force F produced by a linear motor is derived as follows: EQU F=d.phi./dX.multidot.I
That is, the thrust force F produced by a linear motor is proportional to the rate of change in location d.phi./dX of magnetic flux .phi. produced by linkage against winding.
Accordingly with reference to the case, for example, of the slider-and-stator structure identical to that shown in FIG. 11 but comprising a linear motor of bipolar permanent magnet type, which is not illustrated, produced torque T (the rate of change in location d .phi./dX of magnetic flux .phi. produced by linkage against winding) is estimated to be simply proportional to magnetic flux B.
The linear motor shown in FIG. 11 is considered here with the same manner as mentioned above. Suppose that, for example, a single turn of winding is wound through a slot S2 and S8 in the direction from upper surface to bottom of the paper, in which FIG. 11 is illustrated, at S2 and in the direction from down surface to top at S8. Speaking of the rate of change in location of flux linkage .phi. against the winding wound from the slot 2 to 8 in this case, represented by d .phi./dX.apprxeq./.DELTA..phi./.DELTA.X, infinitesimal movement of the slider of .DELTA.X to the right causes infinitesimal change in magnetic flux .DELTA..phi. as increase in magnetic flux of the pole N corresponding to the infinitesimal change in position .phi.X on each tooth between the slot 2 and 8 of the slider. As a result, a high rate of change in magnetic flux d.phi./dX is generated by the position of the winding. Accordingly, the rate of change in location d.phi./dX of magnetic flux .phi. would show fivefold or sixfold increase as compared to the above-mentioned linear motor of bipolar permanent magnet type in simple theory and ditto for generated thrust force. As has been said, a vernier-type linear motor is characterized by generation of high thrust through a principle thereof. However, the effects by limitations of driving frequency and inductance of winding, in general, make high-velocity drive of the linear motor difficult because the driving frequency for controllable drive in this example becomes approximately 6 times.
The linear motor shown in FIG. 11 has a problem that effective use has not been made of magnetic flux of respective permanent magnet. Taking a look, for example, at magnetic flux occurred on the protruding pole of the slider located between the slot S2 and S3, the protruding pole of the slider faces the magnetic pole N of a permanent magnet over a slight gap. This leads magnetic flux of the pole N to be appeared on the protruding pole of the slider, which is, at the same time, influenced by many components which close magnetic flux between the magnetic pole N and the pole S because of leakage flux from the adjoining pole S at each side of the pole N through a nonmagnetic part such as the gap between protruding poles of the slider. The portion of magnetic flux, which closes the magnetic pole S within magnetic flux of the magnetic pole N, is not utilized in a driving operation. Accordingly, the impossibility of making full use of magnetic flux of the magnetic pole N on the protruding pole of the slider located between the slot S2 and S3, and the same on the other respective protruding poles of the slider causes that effective magnetic flux is not obtained enough. As a result, this produces a problem that the thrust of a motor decreases even when the motor is appropriately energized.
The leakage flux of the magnetic pole S is explained below through the magnetic properties of a permanent magnet. B0 to H0 indicated in FIG. 12 is a characteristic example as to typical magnetic flux density B and magnetomotive force H of a rare-earth magnet. The operating point of the pole N of a magnet opposing to the protruding pole of the slider between the slot S2 and S3 is OP1. At this point, a magnetomotive force H1 mainly functions as the magnetomotive force to an air-gap part and its magnetic flux density is B1. On the other hand, the operating point of the adjoining magnetic poles S of each side of the pole N is OP2. At this point, as leakage flux in a nonmagnetic part such as a gap between the slot S2 and S3 is generated, a high magnetomotive force H2 acts and magnetic flux density therein is B2. The portion of B2 closes magnetic flux between adjoining magnets and does not function effectively for a linear motor. As a consequence, magnetic flux working as an action of a linear motor is that equal to the magnetic flux density of B1 minus B2. Depending on the structures of a linear motor the value of magnetic flux varies, but the value of B2 may often be more than the half of B1 so that the half portion of the power of a permanent magnet can not be utilized in many cases.
One of the other problems is that maximum magnetic flux density in a magnetic pole part on the stator is only 1.0 tesla when a rare-earth magnet, whose residual flux density is high, is used, as opposed to that in magnetic steel of a protruding pole on the slider is high as 1.7 tesla, and limitations in structure of the stator interfere with increasing magnetic flux density. It has been expected an increase of motor torque according to an increase of magnetic flux density in each magnetic pole of the stator.
Another problem is that if the driving range of a linear motor is long, the cost of a linear motor becomes high because an expensive permanent magnet attached on a stator requires the amount being proportional to the driving range.
And the other problem is that if a linear motor is applied to a feed driver of a machine tool and so on, the place attached a permanent magnet thereon needs to be strictly covered with consideration given to avoid adhesion of steel dust existing in an ambient environment. This also causes an increase of the cost spent on cover.